The present invention is platinum group-metal curing liquid silicone rubber compositions having improved physical properties such as tear and heat aging, especially at durometers below about 35 and above about 65. The present inventors have unexpectedly discovered that the addition of diorganohydrogensiloxy end-blocked polydiorganosiloxanes and triorganosiloxy end-blocked polydiorganosiloxanes having a viscosity greater than about 10 Pa.cndot.s at 25.degree. C. to liquid silicone rubber compositions can provide for the described improved properties.
Polmanteer et al., U.S. Pat. No. 3,967,473, describe a composition which is fluid and consists essentially of a polydiorganosiloxane having two vinyl radicals per molecule and terminated with triorganosiloxy groups, a mixture of silicone compounds containing silicon-bonded hydrogen atoms, a platinum catalyst, and optionally a reinforcing silica. The cured elastomers prepared from the compositions described by Polmanteer et al. can have a low durometer, but typically the tear strength is poor and below about 26 kN/m (150 ppi). In addition, the compositions described by Polmanteer et al. are unsuitable for making high durometer silicone elastomers with good tear strength and heat stability.
Jeram et al., U.S. Pat. No. 3,884,866, discloses curable organosiloxane compositions having improve physical properties which comprises (A) a high viscosity component containing vinyl-terminated polydiorganosiloxane having a viscosity of from 5 Pa.cndot.s to 1,000 Pa.cndot.s at 25.degree. C., and (B) a low viscosity component containing at least one polydiorganosiloxane having a viscosity of from 0.05 Pa.cndot.s to 5 Pa.cndot.s at 25.degree. C. The curable compositions also contain an organohydrogensiloxane crosslinker and a platinum hydrosilation catalyst.
Jensen, U.S. Pat. No. 4,753,978, teaches that the tear strength of platinum curable liquid silicone rubbers as described by Jeram et al., supra, can by improved if the compositions contain a mixture of two liquid diorganovinylsiloxy terminated polydiorganosiloxanes. One of the diorganovinylsiloxy terminated polydiorganosiloxanes comprises 70 to 95 percent by weight of the mixture and contains vinyl or other ethylenically unsaturation hydrocarbon radicals only at the terminal positions and has a viscosity of from 20 Pa.cndot.s to 200 Pa.cndot.s at 25.degree. C. The second polydiorganosiloxane contains both terminal vinyl radicals and from 1 to 5 mole percent of vinyl radicals on non-terminal repeating units and has a viscosity of from 0.1 Pa.cndot.s to 200 Pa.cndot.s at 25.degree. C. Although the compositions taught by Jensen, supra, cure to form elastomers having excellent tear strength at durometers above about 35, the tear strength tends to deteriorate at lower durometers and is typically less than about 26 kN/m (150 ppi). Furthermore, the cured compositions taught by Jensen, supra, at durometers above about 65 have poor heat aging properties, tending to become brittle.
Takamizawa et al., U.S. Pat. No. 4,057,596, teach an anti-sticking silicone compositions comprising (a) a polydiorganosiloxane of substantially straight-chain molecular structure having dimethylvinylsiloxy terminal groups at both ends and preferably having a viscosity within a range of 0.05 Pa.cndot.s to 5 Pa.cndot. at 25.degree. C., (b) a polydiorganosiloxane of substantially straight-chain structure having dimethylhydrogensiloxy groups at both chain ends and a preferred viscosity of 0.002 Pa.cndot.s to 0.1 Pa.cndot.s at 25.degree. C., (c) a polymethylvinylsiloxane having at least three vinyl groups bonded to the silicon atoms in one molecule, (d) a polymethylhydrogensiloxane having at least three hydrogen atoms bonded directly to the silicon atoms in one molecule, and (e) an addition reaction catalyst.